Holey graphene/MnO2 nanosheets with open ion channels for high-performance solid-state asymmetric supercapacitors

Holey graphene/MnO₂ (HG/MnO₂) composites with open ion channels are synthesized by an electrostatic self-assembly method. The HG with rich in-plane nanopores is prepared via a mild etching reaction first, followed by modified with poly-diallyldimethylammoniumchloride (PDDA) to transfer the surface charge of HG nanosheets from negative to positive, which are eventually assembled on the negatively charged MnO₂ nanosheets via electrostatic attraction. As a result, the HG allows ions to pass through the graphene sheet, improving the ion transport channels. Besides, the electrostatic self-assembly between HG and MnO₂ enables the composite a high conductivity, providing effective electron transport pathways. The HG and MnO₂ sheets are observed to be tightly bounded by the transmission electron microscope (TEM), and the HG content in the composites is determined to be 9.6% to 20% by the thermogravimetric (TG) test. The HG/MnO₂-2 electrode with the HG content of around 14.8% displays the large specific capacitance of 219.3 F g⁻¹ at 0.5 A g⁻¹ and the high rate capacity of 134.7 F g⁻¹ at 10 A g⁻¹. Furthermore, the as-prepared solid-state asymmetric supercapacitors (SSAS) achieve a wide stable operating voltage of 1.8 V, a high energy density of 16.8 Wh kg⁻¹ at the power density of 224.6 W kg⁻¹, and low capacitance degradation of only 6.3% after 5000 cycles.